1. Field of the Invention
The present invention concerns a method for estimating the motion of at least one target in a sequence of images, the motion of this target and the motion of the background of the images being comparable to translations. The estimation of the motion of a target enables the detection and tracking of this target, despite the motion of the background of the images due to the shifting of the vehicle carrying the image sensor, and despite the shifting of the target in relation to the background of the images.
2. Description of the Prior Art
The detection and tracking of a target can be done according to two known methods:
either the target is modelized and attributes defining it are computed. The detection then making a search, in the image, for a zone defined by attributes of little difference compared with those of the target;
or the target is defined as being a zone of pixels in motion between a reference image and the current image.
These two methods can be used independently or simultaneously. When the attributes proper to the target cannot be determined on an a priori basis, the second method is implemented. It necessitates, first of all, the resetting of the background of the current image in relation to the background of the reference image. It then consists in extracting the target in computing the differences in luminance between the reference image and the current image after the resetting of its background. The pixels having a difference in luminance that is greater than a fixed threshold value are considered to have a true motion, due to the presence of a target in motion, while the pixels having a difference in luminance below the threshold value are considered to have a motion due to noise.
In practice, the images are often greatly noise-infested, for example because they are supplied by infra-red image sensors. The noise makes it difficult to estimate the background motion as well as to detect and estimate the motion of one or more targets.
C. B. KUGLIN and D. C. HINES give a description, in IEEE 1975, International Conference on Cybernetics and Society, San Francisco 1975, of a method for estimating the translation motion of the background in a sequence of images using a phase correlation. It consists in sampling the current image and the background image, computing, for each of these two images, a matrix of coefficients transformed by the two-dimensional discrete Fourier transform, computing a so-called phase difference matrix, each coefficient of which is equal to the product of a coefficient of the first matrix by the conjugate of a coefficient of a second matrix divided by the modulus of this product, computing a matrix called an inverse transformed matrix of the phase correlation matrix, applying the inverse Fourier transform to the phase correlation matrix, classifying the coefficients of the inverse transformed matrix according to their modulus, estimating a vector of translation of the background of the images on the basis of the order of the row and the order of the column in which there is located the coefficient having the greatest modulus in the inverted transformed coefficient of the phase correlation matrix.